We propose a solid-state single-photon avalanche detector (SPAD), based on large band gap compound semiconductor materials and a new Geiger-mode avalanche photodiode structure. It will be able to detect light levels down to lone photons with uniquely low background dark noise and high quantum efficiency at room temperature. It will also detect photons showered by a scintillating material from radioactive particle emissions. The SPAD will have superior performance to vacuum phototubes and all previous APDs. The SPAD can make many important biomedical instruments orders of magnitude lower cost, smaller, and more rugged. Fundamental advantages in areas like nanotechnology sensors, PET scanning, MEMS, flow cytometry, and fluorescence microscopy will be enabled by the massive decrease in system footprint and cost offered by this project. [unreadable] [unreadable]